1. Field of the Invention
This invention relates to a propylene polymer composition stabilized against oxidative degradation comprising (1) a propylene polymer, (2) a poly(tertiary-butylated p-vinyl phenol) as a stabilizer, and (3) a dialkylthiodipropionate as an auxiliary stabilizer.
2. Description of the Prior Art
Polypropylene is a crystalline polymer having a high molecular weight and a high degree of stereoregularity. Because of its high melting point and high strength, polypropylene has found extensive applications as films fibers, tubings and molded articles.
Polypropylene has a number of hydrogen atoms on tertiary carbon atoms in the polymer chain. The hydrogen atoms on the tertiary carbon atoms are very reactive, and are a cause of oxidative degradation. Furthermore, polypropylene frequently contain catalyst residues containing titanium and aluminum employed at the time of polymerization, and these catalyst residues catalytically accelerate the degradation of polypropylene.
Accordingly, a great need exists for stabilizers capable of inhibiting the degradation of polypropylene. The previous practice has been to employ stabilizers effective for polyethylene to polypropylene. However, since polypropylene is fabricated at far higher temperatures than polyethylene, many stabilizers effective for polyethylene do not always exhibit stabilizing effects on polypropylene. Many of these stabilizers are effective only for short periods of time, or are hardly effective at all. For this reason, the use of polypropylene has been markedly restricted despite the superior properties of polypropylene.
Of the stabilizers heretofore used, phenolic anti-oxidants such as 2,6-di-tert.-butyl-p-cresol and 4,4'-n-butylidene-bis(2-tert.-butyl-5-methylphenol) have relatively great effects, and are used as additives for polypropylene resins. Since, however, the processing temperature for polypropylene resins is high, these compounds volatilize during processing, or are easily extracted from the resins by water, oils, etc. As a result, these compounds are capable of stabilizing the resins only for a short period of time. Attempts have also been made to improve the resistance of these stabilizers to volatilization and extraction by increasing their molecular weight, and a low-molecular-weight condensate between phenol and formaldehyde has been suggested, e.g., as disclosed in Die Makromolekulare Chemie, 152, 105-120 (1972). This condensate, however, has only a low stabilization effect, is not capable of stabilizing synthetic resins over long periods of time, and colors the resins. In particular, polypropylene resins susceptible to oxidative degradation are difficult to stabilize with these compounds.
British Pat. No. 971,753 discloses the addition of a poly(alkylated alkenyl phenol) to rubber as an antioxidant. However, poly(alkylated alkenyl phenols) do not have a great stabilizing effect when they are added alone to rubber or a resin.